Particles for use as toners in electrophotographic processes must fulfil several requirements in order to function in the intended manner. The particles must be suffieiently hard or they will break down by abrasion during the treatment in the apparatus and also to prevent caking at ordinary operation conditions. On the other hand, the particles must have a sufficiently low melting point to be fixed to the support by conventional methods such as rolling or application of heat. The particles must have suitable magnetic or electrical properties, and particularly the surface must be capable of accepting and retaining the required charge and, especially in cases when the charge is obtained by rubbing against other materials, it must have stable triboelectrical properties. The surface must, however, not be hygroscopic so that water is retained as this can lead to caking of the particles or to a change in the electrical properties. The material of the particles must be resistant to aging and it must be possible to incorporate an aging resistant colorant. The particles should also have as uniform and spherical form as possible to increase the abrasion resistance, to prevent the particles from being fixed to non-desired spots on the charged surface and to make a higher charge, and a charge which is more uniformly distributed between the particles, possible. The particles should also have as uniform size distribution as possible since variations in the size leads to variations in the capability of accepting the charge and thus to a non-uniform deposition on the accepting surface so that the resolution becomes limited.
Known toner materials have only fulfilled the above mentioned requirements to a limited extent. The usual method of preparation, incorporation of a colorant, charge-modifying agents etc. in a plastic mass followed by grinding and screening of the product, certainly makes it possible to choose the material and the additives fairly freely but the obtained particles are of very varying shapes and have a very non-uniform size distribution and this leads to poor abrasion resistance, poor resolution and difficulties in removing the particles, particularly the smaller ones, from non-desired spots on the surface. Alternative methods for preparing toner particles have been suggested and these methods have given particles of a more rounded form, but these known methods have not given the desired narrow size distribution, they have decreased the freedom of choice as concerns the material and the incorporation of additives has been made more difficult.
The preparation of spherical polymer particles having an extremely uniform size distribution by a swelling process to sizes suitable for toner particles is previously known. This method is described in the European patent application 3 905, which is incorporated by reference herein. However, the conditions at the swelling are sure that conventional methods for coloring and adaption of the properties in other respect to the toner-use cannot be used or carried out quite easily.